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Flashes from the beginning of the Universe

By Marcus Chown

THE dark matter that is thought to make up most of the mass of the Universe could be made to reveal itself by creating tiny flashes of light as it passes through a huge underground tank of inert gas, two physicists in the US suggest. This will only happen, however, if the dark matter is made of hypothetical particles called neutralinos.

These particles are predicted by the theory of supersymmetry, which views the two main classes of subatomic particles – fermions and bosons – as opposite sides of the same coin. The difference between fermions and bosons lies in a fundamental characteristic called spin.

Supersymmetry says that every boson has a fermionic partner, and vice versa. Bosons such as photons and gluons are twinned with photinos and gluinos, while fermions such as electrons and quarks are partnered by selectrons and squarks. The as yet undiscovered supersymmetric partners to the known fermions and bosons would have been created in the big bang along with ordinary particles.

In the simplest theories, the lightest supersymmetric particle – the neutralino – is stable, and so could still be around. This makes it a strong candidate for the mysterious particles that are thought to compose about four-fifths of the mass of the halo of dark matter that surrounds our Milky Way (New Scientist, Science, 29 April).

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The neutralino is unusual among supersymmetric particles in that it does not have a direct partner in the family of ordinary particles. Instead, it shares the characteristics of three other supersymmetric particles&colon; the photino, the Higgsino and the Z-ino.

If the dark halo is composed largely of neutralinos, up to 100 million of them could pass through each square centimetre of the Earth’s surface every second. Since they are a type of weakly interacting massive particle, or WIMP, most will pass straight through the Earth. But David Spergel of Princeton University in New Jersey and Glenn Starkman of Case Western Reserve University in Ohio say there is a small chance that as a neutralino passes through ordinary atoms it will exchange a selectron with an electron, boosting the electron to a higher energy state. As the excited electron gives up this excess energy, it should release a photon of ultraviolet light.

Current neutralino detectors rely on a different type of interaction. They look for the recoil of an atomic nucleus after a neutralino and nucleus exchange a squark. These detectors use crystals of a semiconductor such as germanium, in which a recoiling nucleus should eject electrons from neighbouring atoms and cause vibrations of the crystal lattice known as phonons. In principle, it should be possible to detect these electrical disturbances and phonons, but the practical problems of distinguishing them from all the other interactions taking place in the crystal lattice are enormous.

Because the neutralino-electron interaction emits light, it should be much easier to detect. It has been ignored up till now because it was thought to be a million times less likely than the interaction between a neutralino and a nucleus. However, Starkman and Spergel have re-examined the process and say that several factors combine to make a neutralino-electron event just as likely as a neutralino-nucleus interaction (Physical Review Letters, vol 74, p 2623). Previously, for instance, researchers have overlooked the fact that a neutralino can excite any of several electrons in the outer shell of an atom to a number of different possible energy states.

Even so, the proposed neutralino detector would have to be very large to boost the likelihood of an interaction. It would have to be transparent, so that ultraviolet photons could be seen, and made of material with no natural radioactivity. Spergel and Starkman have not considered in detail what materials would be suitable, but one possibility is an inert gas such as helium or argon. To avoid picking up cosmic rays, the detector would be built deep underground.

Spergel says the signature of neutralino-generated ultraviolet photons would be unmistakable. It should show a characteristic yearly variation, reaching a peak in June, when the velocity of the Earth round the Sun is parallel to the velocity of the Sun round the Galaxy, maximising the Earth’s velocity through the Galaxy’s halo.